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Thermostat power

In the year I've owned my house, I've had to replace the upstairs t-stat batteries about 4 times, and never the downstairs. This spoke to me that it was either something wrong with the actual control unit, or the HVAC power wire? Recently it gave me a "bat lo" message in the middle of the night, shutting off the A/C and waking me up. I shook it and it resumed normal operation.

Is there a simple way to test if it's either the unit or wiring? Is it as simple as taking a voltmeter to the wire? What voltage should I be looking for? Sorry if this seems DIY, I just don't have a good understanding about how t-stats are powered and what I should be looking for.

The t-stats for both downstairs and upstairs are Honeywell programmables, at least 12 yrs old.

In the year I've owned my house, I've had to replace the upstairs t-stat batteries about 4 times, and never the downstairs. This spoke to me that it was either something wrong with the actual control unit, or the HVAC power wire? Recently it gave me a "bat lo" message in the middle of the night, shutting off the A/C and waking me up. I shook it and it resumed normal operation.

Is there a simple way to test if it's either the unit or wiring? Is it as simple as taking a voltmeter to the wire? What voltage should I be looking for? Sorry if this seems DIY, I just don't have a good understanding about how t-stats are powered and what I should be looking for.

The t-stats for both downstairs and upstairs are Honeywell programmables, at least 12 yrs old.

Yes, this is DIY and any replies regarding troubleshooting are against the rules. Sorry.

I've only owned my house for a year, but the t-stats are from when it was built (2000). It's been happening on and off over this last year - it seems to drain the batteries in the upstairs unit and the bottom one has been fine. So I guess to rephrase my question, away from DIY is, how are thermostats powered? Is it primary battery with HVAC power backup or vice versa?

I've only owned my house for a year, but the t-stats are from when it was built (2000). It's been happening on and off over this last year - it seems to drain the batteries in the upstairs unit and the bottom one has been fine. So I guess to rephrase my question, away from DIY is, how are thermostats powered? Is it primary battery with HVAC power backup or vice versa?

Depends on the model thermostat, and how it is wired. Some are battery powered only, some are either battery or hardwired, and some are system powered with or without battery backup.

Consider upgrading to a system powered digital thermostat. These aren't sold at the big box stores and require a common wire connection so you will need a pro to hook it up for you. IMHO all digital stats should be common wire powered, but unfortunately most builders are cheap and use the battery powered stats.